1. Field of the Invention
This invention relates to magnetic recording disk drives, and more particularly to disk drives that have a thermal fly-height actuator for controlling the spacing between the read/write head and the disk.
2. Description of the Related Art
Magnetic recording hard disk drives use a read/write transducer or head mounted on a head carrier for reading and/or writing data to the disk. The head carrier is typically an air-bearing slider attached to an actuator arm by a suspension and positioned very close to the disk surface by the suspension. There are typically a stack of disks in the disk drive with a slider-suspension assembly associated with each disk surface in the stack.
The separation or spacing between the head and the disk surface is called the fly-height. The slider has a disk-facing air-bearing surface (ABS) that causes the slider to ride on a cushion or bearing of air generated by rotation of the disk. The slider is attached to a flexure on the suspension and the suspension includes a load beam that applies a load force to the slider to counteract the air-bearing force while permitting the slider to “pitch” and “roll”. The flying dynamics of the slider and thus the fly-height are influenced by factors such as the rotation speed of the disk, the aerodynamic shape of the slider's ABS, the load force applied to the slider by the suspension, and the pitch and roll torques applied to the slider by the suspension.
Disk drives may use a fly-height actuator for changing the spacing between the head and the disk surface. One type of fly-height actuator is a thermal actuator with an electrically-resistive heater located on the slider near the head. When current is applied to the heater the heater expands and causes the head to expand and thus move closer to the disk surface. Disk drives with thermal fly-height actuators typically adjust the fly-height to different heights, depending on whether the drive is reading or writing. Also, the fly-heights may be adjusted depending on the radial location, i.e., the data track, where the head is reading or writing data. Thus there are typically a set of different heater power values that can be applied, depending on the head selected and the band where data is read or written.
However, even in the absence of heat to the heater, the write head expands during a write operation as a result of heat from the write coil, which results in protrusion of the write head pole. Thus the fly-height of the write head decreases exponentially during a write operation. To compensate for this, the heater power can be adjusted (decreased) to increase the fly-height. However, if the nominal fly-height cannot be maintained data errors can occur during writing or the write head can fly too close to the disk, causing premature mechanical head wear.
What is needed is a disk drive with a thermal fly-height actuator and a controller that accurately compensates for write head expansion during writing.